1. Field of the Invention
The present invention relates to an ion selective electrode and more particularly to an ion selective electrode which has a polymer material as a support and which is suitable for use in analysis of anions such as chloride ion, and cations such as alkali metals and alkaline earth metals, which are contained in living organisms, foods or the environment.
2. Description of Related Art
An ion selective electrode is used together with a reference electrode, and when both are allowed to contact, for example, body fluid, an electromotive force depending on the objective ion concentration is generated between the electrodes and an operational processing is carried out based on the output power to measure the objective ion concentration. The ion selective electrode used in this way includes a solid electrode comprising silver chloride and silver sulfide and a polymer supporting electrode comprising a polymer in which an ion exchanger or neutral carrier is dispersed as a sensitive material. As the latter ion selective electrode which comprises an ion exchanger as a sensitive material, in the conventional chloride ion selective electrode, a sensitive membrane is composed of an ion sensitive material, a plasticizer and a base material. An organic compound having a relatively high dielectric constant of at least 15 is used as a part of the plasticizer in order that the ion sensitive material in the sensitive membrane is dissociated to enhance solubility and to promote ion dissociation equilibration, whereby the electrode is operated as a liquid membrane type ion selective electrode as disclosed in Japanese Patent Kokai (Laid-Open) No. 59-137851. However, investigation of the organic compound as a plasticizer used to promote ion dissociation equilibration of an ion exchanger which is a sensitive material in the sensitive membrane has been insufficient. Therefore, when the conventional plasticizer is used, impedance of the liquid membrane type ion selective electrode which contains a straight chain alcohol which readily crystallizes at low temperatures increases in use over a long period of time and the response speed is apt to decrease.
Furthermore, as a chloride ion selective electrode used for analysis of chloride ions, for example, in living body fluid, there has been employed a solid membrane type electrode comprising silver chloride and silver sulfide or a polymer supporting liquid membrane type electrode in which an ion exchanger is supported as a sensitive material in a synthetic polymer supporting membrane such as polyvinyl chloride. The former electrode suffers from the problem that obstruction by halogen ions such as bromide ion or sulfide ions is great. On the other hand, the latter electrode also suffers from the problems that errors in measurement are apt to occur due to oleophilic ions trapped in the sensitive membrane and life of electrode is shortened due to a dissolving out of the ion exchanger used as a sensitive material from sensitive membrane or adsorption of protein and the like to the surface of sensitive membrane. Under the circumstances, attempts have been made to solve these problems with polymer supporting liquid membrane type electrodes. For example, Japanese Patent Kokai (Laid-Open) No. 56-63246 proposes the use of methyltridodecylammonium chloride as a sensitive material and n-tetradecyl alcohol (straight chain aliphatic alcohol of 14 carbon atoms) as a plasticizer in order to reduce obstruction due to adsorption of protein and the like. Japanese Patent Kokai (Laid-Open) No. 59-137851 has proposed to use dimethyldioctadecylammonium chloride as a sensitive material and a mixture of a straight chain aliphatic alcohol and o-nitrophenyloctyl ether as a plasticizer in order to improve responsivity and reduce errors in measurement. Furthermore, it has been disclosed that n-dodecyl alcohol (carbon number: 12 and melting point: 23.5.degree. C.) is added in addition to n-tetradecyl alcohol (carbon number: 14 and melting point: 38.3.degree. C.) as the straight chain aliphatic alcohol in order to increase solubility of the sensitive material and decrease crystallization of the membrane material. Moreover, according to the process disclosed in Mikrochimica Acta, (Wein) 1984 III, 1, tetradodecylammonium chloride is used as a sensitive material in order to improve selectivity over hydrogen-carbonate ion contained in blood. However, the chloride ion selective electrode in which the above sensitive material is used is deteriorated in selectivity over oleophilic anion. As mentioned above, conventional methods for improvement have tried to solve the problems by changing the structure or kind of sensitive material and plasticizer. However, in the case of a chloride ion selective electrode according to the conventional methods, if it is attempted to control the selectivity over oleophilic anions, selectivity over the hydrophilic anions such as hydrogencarbonate ions decreases. On the contrary, if it is attempted to improve the selectivity over hydrophilic anions such as hydogencarbonate ions, selectivity over oleophilic anions is extremely deteriorated. Therefore, in order to reduce errors in measurement and to improve the accuracy of a polymer supporting liquid membrane type chloride ion selective electrode, it is important to improve selectivity over both the hydrophilic anions and the oleophilic anions. It is also important for functioning as a liquid membrane type electrode to select a plasticizer which can increase the solubility of the sensitive material and reduce the crystallization of the membrane material, and which is stably maintained in the sensitive membrane.
In the conventional techniques, namely, in the conventional polymer supporting liquid membrane type chloride ion selective electrode where a straight chain aliphatic alcohol is used as a plasticizer, examination on carbon number of the aliphatic alcohol has been insufficient. Especially when tetraalkylammonium salt is used as a sensitive material, there is the problem that if the carbon number of aliphatic alcohol which is a plasticizer is increased, the aliphatic alcohol is readily crystallized and dissolves the sensitive material with difficulty and thus the impedance of the electrode increases resulting in a decrease of response speed. On the other hand, there is also the problem that if the carbon number of the aliphatic alcohol is decreased, water-solubility thereof increases and hence the response to potential becomes unstable and the performance of the electrode cannot be maintained for a long time. As mentioned above, when a dimethyldioctadecylammonium salt is used as a sensitive material, there is the known technique (Japanese Patent Kokai (Laid-Open) No. 59-137851) which uses mixtures of straight chain aliphatic alcohols different in carbon number as a plasticizer, but it is difficult to apply this known technique in the case of using a tetraalkylammonium salt which is very low in solubility as a sensitive material. This is because the known technique uses aliphatic alcohols different by two in carbon number and so the difference in properties of them is too great to solve all of the above problems.